1. Field of the Invention
The present invention relates to a chemical process for making useful cationic vinyl monomers and, more particularly, pertains to an improved non-catalytic process for the preparation of N-(tertiaryaminoalkyl)acrylamides having very low acid impurity levels. The products of this invention are useful in preparing flocculants, adhesion promoters, oil soluble dispersants, epoxy curing agents and ion exchange resins.
2. Description of the Prior Art
The basic process for the production of N-(tertiaryaminoalkyl)acrylamides related to this invention is given in U.S. Pat. No. 3,878,247, in which an acrylic acid or ester is first reacted with excessive tertiary amino alkyl amine to form a .beta.-aminopropionamide intermediate. Acrylic acid or ester compounds useful in the invention include acrylic acid, methacrylic acid, methyl acrylate, methyl methacrylate, ethyl acrylate and ethyl methacrylate. Temperatures from 170.degree. to 210.degree. C. are employed. The propionamide intermediate is then pyrolyzed at 180.degree. to 300.degree. C. to the desired N-(tertiaryaminoalkyl)acrylamide product and to recyclable tertiary amino alkyl amine.
When the process of U.S. Pat. No. 3,878,247 was scaled up to produce substituted acrylamide products like dimethylaminopropyl methacrylamide (DMAPMA), it was found that there was a level of acrylic acid impurity in the product. Potential polymerization applications for products such as DMAPMA require the highest possible product purities. At the present time, utilizing the method of U.S. Pat. No. 3,878,247, attainment of product acid impurity levels of approximately 0.1 meq/gm, (approximately 0.86 weight percent), which is the maximum allowable for many purposes, requires propionamide reactor batch cycles of up to twelve hours. A water removal and purification tower, as well as reactor pressure requirements, for the initial propionamide reactor system used may dictate that the reactor be expensive in cost. In addition, it was thought that a by-product water removal stream had to be vented at all times to drive the reaction sufficiently. This task was accomplished by carrying out the reaction at the bubble point of the reactor contents.
It is an object of this invention to provide a process for the production of N-(tertiaryaminoalkyl)amides which have a very low acid impurity level in the final product stream. It was surprisingly found that holding the batch or continuously produced propionamide intermediate in an atmospheric tank at 150.degree. to 220.degree. C. immediately prior to continuous pyrolysis at 180.degree. to 300.degree. C., rather than cooling down and re-heating the propionamide intermediate before and after storage, as was the usual method described in U.S. Pat. No. 3,878,247, would reduce the acid impurity level of the final product by at least an entire order of magnitude. Holding the propionamide at an elevated temperature not only allows obvious savings in process utility costs, but also allows for the surprising result of a considerable reduction in the acid impurity content of the final product. Further unexpected advantages are that there is no recognizable yield loss under the closely controlled conditions described, and product stability is improved by a lower acid content.
While acid concentration can be reduced by longer hold times in the initial, pressure rated propionamide reactor, the capital costs of a larger initial reactor are very high compared with a lower cost "post reactor" or surge tank, which is essentially an atmospheric pressure rated storage tank with a small vent condenser. The concept of a lower cost "post reactor" surge or storage tank might thereby allow considerable savings in plant capital costs, as well as providing an increase in the final product purity. Other potentially workable methods of reducing the acid impurity level in the N-(tertiaryaminoalkyl)acrylamide product, such as extraction, are potentially more expensive than the improvement described herein.